1. Field of the Invention
The present invention relates to a diamond film field effect transistor using diamond films.
2. Description of the Related Art
Diamond has an excellent stability against heat, and has a wide band gap of about 5.4 eV. It is an electrically insulating material, but becomes a p-type semiconducting material by being doped with atoms of boron (B). In recent years, as a technique of forming diamond films by vapor phase synthesis has been established, heat-resisting electronic devices such as diodes and transistors have been manufactured using the semiconducting diamond films thus obtained.
FIG. 4 shows one example of device structures of metal-insulator-semiconductor field effect transistors (hereinafter, referred to as "MISFET") using monocrystalline diamond films (Unexamined Japanese Patent Publication No. HEI 1-158774). In this figure, a monocrystalline diamond film 3 doped with boron (B) (hereinafter, referred to as "p-layer") is formed on a monocrystalline electrically insulating diamond substrate 1 by chemical vapor deposition. Moreover, an electrically insulating diamond film 4 (hereinafter, referred to as "i-layer") is selectively deposited on the p-layer 3. A source electrode 5 and a drain electrode 6, and a gate electrode 7 are respectively formed on the p-layer 3 and the i-layer 4 with a specified pattern. The film thickness of the p-layer 3 is specified to be about 0.5 .mu.m.
The prior art MISFET shown in FIG. 4, however, has the following disadvantage: namely, since the film thickness and the B-doping concentration of the p-layer 3, and the film thickness of the i-layer 4 are not optimized, the modulation of the current flowing between the source electrode and the drain electrode is very small. The modulation of the source-drain current is a key factor for operational FETs. In addition, the prior art MISFET structure is also disadvantageous in that the transistor characteristics are deteriorated when moisture and the like become adsorbed on the surface of the p-layer 3, providing an alternative path for the source-drain current.